A natural arch is a rock formation with a hole going completely through it. Erosion and other natural processes, working in concert or individually, initially form the arch. Weathering of an arch occurs due to various forms of macroscopic and microscopic erosion. Biological and chemical actions also can weather arches. Many of the forces that cause rock weathering are also responsible for arch formation.
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Microscopic Erosion Through Chemical Exfoliation
Snow, ice or seeping water can permeate an arch's rock, resulting in weathering by chemical exfoliation on a microscopic level. The seeping water dissolves crystalline grains that have held the rock together. Wind, water or gravity removes the loosened particles of the rock. Acids in biological agents also can cause chemical weathering.
An arch can become weathered when fractures in the arch's rock become wider and break off into large pieces, a process known as macroscopic erosion. Earthquakes can cause such fractures and erosion. Thermal exfoliation is defined as macroscopic fractures and breakage due to temperature fluctuations. Water or gravity acts to move the macroscopic pieces away from the arch's main mass. Macroscopic erosion is also known as mechanical weathering.
Wind is a form of microscopic erosion. Intense winds may sandblast rock by picking up grains of sand and pounding them against the rock, thereby smoothing the arch's surface.
A constant flow of water from streams and rivers can cause physical weathering by cutting a path into an arch's rock. The advance and retreat of waves and glaciers also cause physical weathering.
Natural rock arches can weather due to plant or animal growth. These life forms excrete acidic organic materials that etch the rock, and plants grow roots that can cause fractures in the rock. Iron-oxidising bacteria may also weather rock on the microscopic level.
Cold climates with days that hover around the freezing point of water can cause freeze-thaw weathering, in which liquid water flows through cracks in rock and then freezes at night. The expansion of freezing water forces the arch's rock apart, and macroscopic pieces break off. This form of physical weathering is also called frost shattering or freeze expansion.
Warm Climate Exfoliation
Fluctuating temperatures in very warm climates can cause exfoliation of an arch. When rock is heated (to relatively high temperatures) and cooled repeatedly, physical weathering occurs.
Permeable rocks, such as limestone, sandstone and chalk, allow water to pass through them. When permeable rock is eroded by flowing water, an arch resting on it can fall due to gravity alone.
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